Direct injection engines have an open combustion chamber into which fuel is injected directly. In a direct injection engine, such as a diesel or spark ignition direct engine operating with multi-hole nozzles, the injected fuel meets a high-temperature and high-pressure environment. In order to ensure a clean and efficient combustion process, the injected fuel needs to mix rapidly with the surrounding air. This constitutes a particular challenge in situations where excessive external exhaust gas recirculation (EGR) is applied to control emissions of nitrogen oxides (NOx).
Recent interest in fuel efficiency and federally mandated emission requirements has refocused efforts to develop highly efficient, low emission engines and modes of operation. In view of possible future stringent emissions legislation, particularly pertaining to nitrogen oxides (NOx), it is necessary to operate the engine at high EGRs. However, this generally has a negative influence on soot emission due to limited oxygen availability during the combustion process.